1. Field of the Invention
This invention relates to an installation for the study of the surface of samples placed in a vacuum or controlled atmosphere. It applies in particular to electronic tunneling effect microscopy and/or spectroscopy, namely in ultravacuum, to optic tunneling effect microscopy and/or spectroscopy or further to the etching of nanometric structures through optic and/or electronic microlithographical processes.
2. Discussion of Background Information
It is a well known fact that certain microscopy, spectroscopy or microlithography processes can only be conducted in a particularly clean, controlled atmosphere especially in order not to pollute the surface of the samples being studied, or the samples on which tiny structures are to be realized, such as optic or electronic integrated circuits. In this respect, the vacuum is a particularly useful controlled medium and some applications even require the samples and measuring or etching means to be placed in an ultravacuum, i.e., in a high vacuum. In this sense, a series of microscopy or spectroscopy techniques have been developed employing a lateral scanning of the surface of a sample by an electricity or light conducting tip, this tip being an integral part of a microprobe collecting, by tunneling effect, electrons or photons in numbers that largely depend on an exponential function of the distance separating the tip from the surface; it is common usage to call electronic tunneling microscopy, or STM (Scanning Tunneling Microscopy), the technique using electrons, and optic tunneling microscopy, or PSTM (Photon Scanning Tunneling Microscopy) the technique using photons. As a general rule, these two techniques and equivalent techniques, i.e., those in which a microprobe should be positioned close to the surface of a sample to be studied, either in the air or in a vacuum will be collectively referred to as SXM. Moreover, these initials will be used to refer to microscopes or means of study using these techniques. Finally, it should be noted that SXMs all operate in the same way as a roughometer, and that this expression will be generally used without distinguishing the physical principle applied during measurement.
In all cases, the distances to be maintained between the surface and the microprobe or tip are extremely small, i.e., 1-2 nanometers for the STM and approximately one micrometer for the PSTM. Such small values mean that risks of error in manipulation should be reduced to a minimum and therefore require the use of highly reliable mechanical, electronic or optical components. Without such highly reliable components destructive impacts car more or less, occur between the microprobe and the surface of the sample to be studied or etched, these impacts being caused by the loss of control of the spatial position of the tip. Another cause of damage results from too quick a scanning of the surface with regard to the response time of the components or mechanical and/or electronic circuits controlling the distance between the tip and the surface. Moreover scanning speed is an obvious function of the roughness of the surface. This means that a smooth surface can be scanned quicker than a rough surface, but it is never easy to know exactly before the study. In the case of an impact caused for some reason, the microprobe or the tip must be changed frequently, or at least controlled, which is not always possible directly in an enclosure where a vacuum has been made or where a controlled atmosphere is concerned. In addition, the aging of the tips, especially metal tips in the case of STM, or optic tips in the case of PSTM, is a normal phenomenon requiring their replacement. For example, an STM tip might have to be replaced due to a rearrangement of the terminal atoms of the tip or due to capturing a foreign atom or due to a thermal expansion effect. A final case, also very important, for which it is essential to change the microprobe or tips in order to study the surface of a sample is that where the measurements taken with a first microprobe are uncertain or at least surprising enough to have to be confirmed by a second series of measurements. In particular, it is a well known fact that STM only gives an exploitable topographic image of the surface of a sample if the metal tip has a theoretical profile whose effects on the formation of the image are well established, i.e., vis-a-vis the intensity of the tunnel current being set up between the surface and the tip (the exploitation of a "rough" image thus consists in the deconvolution of the image, in the mathematical sense of the term). Because this condition is not always realized, it might be useful to establish successive cartographs of the same surface, using various tips, the ideal being that these tips are available "in situ" so as to avoid any risk of contaminating the surface under examination by placing the sample in a normal, uncontrolled atmosphere, or simply, due to the delay between successive scannings. Indeed, we know that a scrubbed surface placed in a vacuum becomes covered with a coat of oxides or oxygen within about 1 hour after its scrubbing, even if the vacuum is an ultravacuum, i.e., the surface is contained within an enclosure where the pressure is lower than 10.sup.-9 torr.
Few installations are known that are adapted to the study or etching of the surface of samples placed in vacuum by SXM type techniques offering the possibility of quickly changing the microprobe or the tip used for carrying out this study or etching without having to "interrupt" the vacuum to come back to atmospheric pressure, without dismantling the enclosure to reach the microprobe, or without interrupting the study of the sample for an excessive lapse of time. In this respect, an electric tunneling effect microscope developed by the British company W.A. Technology Limited is equipped with a device allowing the replacement of the tip used for measuring, without having to open the ultravacuum enclosure; the mechanisms used for this purpose are however complex and only allow the tip to be replaced. However, damage can also be done to the microscope itself, or part of it, which will still make it necessary for the ultravacuum enclosure to be opened.
Moreover, it is known that the introduction of a sample in an enclosure in controlled atmosphere or an enclosure in vacuum or ultravacuum is not at all easy. In particular, it is essential to preserve as far as possible the tightness and the cleanliness of the enclosures where the microprobe or the tip used for measuring or etching is located which requires a particularly careful study if samples are to be introduced into the study or etching enclosure without risk of "breaking" the vacuum or contaminating the surrounding atmosphere. This problem has not yet been solved with the existing prototypes, or it has been solved, but in a very complex way, which limits the industrial applications of the SXM installations, and, in particular, that of tunnel microscopes operating in controlled atmosphere or in the ultravacuum.